How to save the material in library
To add a material to your library, first you have to save your file with the material you want to add to the library. 

Next, click add material in the menu as pictured below, and it will be added to the library and available for future use. 



The other alternative is to add a new material to the appropriate .blend file in the addon folder. Remember to save it, or it will be lost of course. 

There should be a .blend for each material library the add-on provides as shown below.